1. Field of the Invention
The invention relates to a flexible flat cable and, in particular, to a flexible flat cable with a shield layer that is used as a wiring material of electric and electronic devices such as audio-video devices and office automation devices.
2. Description of the Related Art
In general, a flexible flat cable is widely used as a jumper wire (or a fixed wiring) between circuits in various electric and electronic devices or as a wiring material wired to a movable portion in the electric and electronic devices in place of a flexible printed-wiring board because of its flexibility (or bendability). In recent years, it has been applied to a wiring material for wiring to a print head portion of a PC inkjet printer or a pick-up portion of CD-ROM drive, car navigation or DVD (digital versatile disc) player, etc.
FIGS. 7 and 8 are schematic cross-sectional views showing an example of a conventional flexible flat cable. As shown in FIG. 7, the conventional flexible flat cable 100 is manufactured such that single or plural parallel-arranged conductors 101 as a signal line are formed into a group of conductors, sandwiched by two insulating films 103 with adhesive 102 adhered to the surface thereof, and processed by thermocompression etc. A reinforcing plate 104 for lining each conductor exposed may be provided at both ends of the flexible flat cable 100, as shown in FIG. 8.
On the other hand, for the purpose of magnetic shield, a flexible flat cable with a shield layer formed by coating with a shield material the insulating film 103 of the flexible flat cable 100 shown in FIG. 7 is applied to electric and electronic devices including audio-video devices such as VTR, CD player or DVD player and office automation devices such as photocopier, scanner or printer. For example, the shield material may have a multilayer structure comprised of an adhesive having conductive properties, a metal material having conductive properties, and an insulating film having insulation properties. The adhesive having conductive properties may be generally an adhesive with conductive fine particles called conductive fillers such as Ni or carbon added thereto.
More recently, along with the popularization of digital devices such as liquid crystal display television or plasma television, a wiring material for high-speed and high-capacity transmission is demanded. Therefore, the demand for the flexible flat cable having a shield layer which can be matched to characteristic impedance of the digital device has been increasing. Such a flexible flat cable having a shield layer in which characteristic impedance is possible includes, e.g., a flexible flat cable configured to have a specific conductor width or distance between each conductor (e.g., see JP-A 2002-184245), a flexible flat cable in which an insulating film is formed of a foam insulator, and a flexible flat cable in which an air-containing layer formed of a nonwoven fabric is provided on an outer surface of an insulating film (e.g., see JP-A 2003-31033, JP-A 2005-339833 and JP-A 2008-277254).
In the conventional flexible flat cable described in the prior art documents, etc., an effective means for matching the characteristic impedance is to control a width of a conductor having a rectangular cross section (a flat shape) or a distance between each conductor, or to apply a foam insulator, etc., having low dielectric constant. However, since flexibility of the flexible flat cable may be insufficient by these means, it is not necessarily possible to satisfy the demand for the flexible flat cable accompanied with downsizing and space saving of the latest electric and electronic devices.
For example, in recent years, accompanied with downsizing and space saving, etc., of the electric and electronic devices, when a flexible flat cable is wired to an electric and electronic device, there is a case that the flexible flat cable is bent 180° and is wired while maintaining the shape. However, the conventional flexible flat cable does not have sufficient flexibility to maintain a 180-degree bent shape, thus, there is a problem that, even though it is bent, it is not possible to maintain the bent shape. Particularly in a flexible flat cable having a shielded layer, there is concern that the shield layer causes a decrease in flexibility.